Iraq's Prime Minister has said that his forces have started to clear cities of "terrorists", as President Barack Obama said he will not be sending troops back to the country to quell an insurgency by Islamists.

It comes after a representative of the country's most senior Shi'ite cleric urged people to take up arms and defend the nation from militants.

Fighters from the Islamic State of Iraq and Syria (ISIS) have been storming across the northern part of the Iraq, forcing the country's US-trained security services to scatter and flee.

They are now thought to be less than 50 miles (80km) from the gates of the capital Baghdad.

Sheikh Abdulmehdi al Karbalai, a representative for Grand Ayatollah Ali al Sistani, said: "People who are capable of carrying arms and fighting the terrorists in defence of their country ... should volunteer to join the security forces to achieve this sacred goal."

Image:Volunteers who have joined the Iraqi Army to fight ISIS board an army truck

Iraq's Prime Minister Nouri al-Maliki travelled to the embattled city of Samarra on Friday for a security meeting. The city was attacked by militants from ISIS earlier in the week.

He acknowledged that the Iraqi troops that had been trained by US forces had not coped when pressed, saying it appeared that there is a "problem".

But he added that it was important to support Iraq and he and his advisers would be spending the next few days working on what could be done to stop ISIS's advance.

The Iraqi government has implemented an emergency plan to defend Baghdad.

Insurgents took control of the northern cities Mosul and Tikrit earlier in the week, having previously seized Fallujah and parts of Ramadi.

The militants have now also moved into the towns of Saadiyah and Jalawla in the eastern province of Diyala, after security forces abandoned their positions.

The Iraqi air force has hit back, launching airstrikes on militant fighters' positions around Mosul and Tikrit.

Video:Iraq Army Flee Militant Advance

As the chaos spread, Iraqi Kurdish forces took control of Kirkuk, an oil hub close to their autonomous enclave, after government troops abandoned their posts.

ISIS has published Sharia rules for the territory it has gained in northern Iraq, including a ban on drugs, alcohol, cigarettes and an edict on women to wear all-covering, shapeless clothing.

Militants are reported to have executed soldiers and policemen after seizing some towns.

UN human rights spokesman Rupert Colville said the number of people killed in Mosul may run into the hundreds.

He said: "We've also had reports suggesting that the government forces have also committed excesses, in particular the shelling of civilian areas. There are claims that up to 30 civilians may have been killed."

General Lord Richard Dannatt, former chief of the general staff, told Sky News the "bitter" conflict is "almost a fight for the soul of Islam itself".